Engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous and solid material, including particulate matter, nitrogen oxides (“NOx”), and sulfur compounds.
Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent over the years. The amount of pollutants emitted from an engine may be regulated depending on the type, size, and/or class of the engine. One method that has been implemented by engine manufacturers to comply with the regulation of particulate matter, NOx, and sulfur compounds exhausted to the environment has been to remove these pollutants from the exhaust flow of an engine with filters. However, using these filters for extended periods of time may cause the pollutants to build up in the components of the filters, thereby causing filter functionality and engine performance to decrease. For example, some engine exhaust systems may include one or more NOx absorbers useful in removing NOx from an exhaust flow. In addition to storing NOx, the alkali metals, alkali earth metals, and/or rare earth metals of the NOx absorber catalyst may also store sulfur compounds. During this process, sulfur present in the exhaust flow is oxidized and the resulting ions are absorbed by the metals of the NOx absorber catalyst to form stable sulfates. These sulfates have a higher binding affinity for the metals of the catalyst than do nitrates and will reduce the number of sites available for NOx absorption within the NOx absorber over time, thereby reducing the effectiveness of the NOx absorber. Thus, in order to improve the performance of the NOx absorber, it may be necessary to periodically purge the sulfur compounds stored therein. The process of removing sulfur compounds from a NOx absorber will be referred to herein as “desulfation.”
One desulfation method may include injecting a reductant, such as diesel fuel, into an exhaust flow at an elevated temperature. For example, U.S. Pat. No. 6,779,339 (“the '339 patent”) to Laroo et al., describes a method and apparatus for desorbing sulfates from a NOx absorber in a fuel rich environment. The system of the '339 patent includes two exhaust flow legs, each including a NOx absorber and a particulate filter. Once a desired desulfation temperature is reached in the NOx absorber of the leg requiring desulfation (“the desulfation leg”), the desulfation leg is closed to exhaust flow and all of the exhaust flow is passed through the nondesulfation leg. Reductant is then sprayed into the particulate filter of the desulfation leg, and the injection is controlled to maintain a desired air-to-fuel ratio (“lambda”). If the NOx absorber catalyst's temperature decreases below optimal desulfation temperatures as sulfur is released therefrom, exhaust flow is once again allowed into the desulfation leg to initiate an exothermic reaction across the particulate filter. The heat given off is then convectively transferred to the NOx absorber to increase its temperature.
Although the filter system of the '339 patent may assist in removing sulfur from the NOx absorber, the method of the '339 patent does not continuously pass the engine exhaust through the desulfation leg during desulfation or actively control the desulfation temperature by varying the exhaust flow rate. Instead, the desulfation leg is closed to exhaust flow when the desired desulfation temperature is reached in the NOx absorber. As a result, the method of the '339 patent provides a variable desulfation temperature that may decrease below optimal desulfation temperatures during sulfur release. Moreover, the disclosed desulfation process may degrade parts of the system due to the high temperatures created and the absence of exhaust flow through the desulfation leg during the process.
The filter system of the present disclosure is directed to overcoming one or more of the problems set forth above.